Renewable fuels are environmentally desirable, especially fuels with higher energy density such as biodiesel. U.S. biodiesel production capacity however, is underused. From January of 2014 to July 2016, annual production of biodiesel did not exceeded 60% of plant capacity. This was due in part to a lack of inexpensive feedstock supply, which the industry greatly needs as an alternative source of seed oils (Anon 2016).
Plant triglycerides from soybeans, canola, rapeseed and palm oil can be converted into methyl or ethyl esters for use as biodiesel. Biodiesel produced in the U.S. is mainly derived from waste cooking oils, the edible oils of soybean and rapeseed (canola) (Hammond et al. 2005), from the inedible corn oil recovered following fermentation and distillation of grain to ethanol, and with lesser amounts generated from animal fats.
If the U.S. were to use all of its domestic soybeans to make biodiesel, it would result in about 5.1 billion gallons of biofuel. This approach, however, is not realistic because these oilseeds are also key components of the food chain and are used for the production of many household and industrial products. Thus, increasing the production of biodiesel from foodstuffs would lead to higher prices of commodities derived from them, and economic hardship for the consumer.
Biodiesel can be made from the triglycerides generated by oleaginous (lipogenic) yeast or algae. In fact, 2 to 3 times more lipid/g dry weight is generated by these microbial sources than from seed oils. Certain algae can accumulate lipids when cultivated on sunlight and CO2, but fixation of CO2 by photosynthesis requires a great deal of metabolic energy, so cell growth and lipid accumulation is relatively slow. Some algae can grow heterotrophically on simple organic compounds dissolved in water, which greatly increases their rates of lipid accumulation. Algae, however, do not generally assimilate more complex organic materials such as starch, cellulosic or hemicellulosic oligomers. Ascomyceteous and basidiomyceteous lipogenic yeasts and filamentous fungi will, however, readily assimilate these compounds. Moreover, because these yeasts and fungi are heterotrophic, their growth rates on simple or complex dissolved organic materials are much faster than algae cultivated under heterotrophic conditions.
If it were possible to produce biodiesel from cellulosic residues or other waste organic materials in yields similar to what could be achieved with ethanol production, domestic biodiesel production could satisfy a significant fraction of the national transport energy demand without affecting the food supply. Furthermore, this increased domestic production would also decrease dependency on foreign oil.
Agricultural residue (e.g. corn stover) is a potential source of renewable biomass that could be converted into liquid transportation fuels such as biodiesel if recalcitrance of the biomass to hydrolysis, the presence of inhibitors mixed with the hydrolyzed sugars, and the difficulty in obtaining microbial catalysts that will convert the sugars to lipids in high yield can be overcome. The potential for biodiesel production from agricultural residues is significant. If the residues from U.S. soybean production alone were collected and converted by a microbial process with a mass yield of 35% based on the starting sugar, it would be possible to produce about 10 million metric tons of lipid annually or about 15% more oil than the total of what is presently recovered from the processing of soybeans itself.
Cellulosic biodiesel produced by a lipogenic yeast cultivated on agricultural hydrolysate would generate an animal feed byproduct similar to that obtained from processing oil from soybeans or palm oil. Based on comparisons with existing prices for wholesale yeast protein from brewing, biodiesel production by lipogenic yeast would yield residual yeast protein with the same or slightly higher market value as soy protein.
While several technologies exist to pretreat and enzymatically saccharify agricultural residues for hydrolysis to create fermentable sugars, new microbial biocatalysts are needed to convert the resulting mixed sugars into lipids and other higher value materials.
Grain ethanol plants are a potential source of unused, soluble and insoluble organic materials suitable for biodiesel production. In wet and dry-mill ethanol operations, cornstarch is enzymatically converted into sugar then fermented to ethanol. The process leaves behind significant amounts of corn fiber and generates soluble organics as byproducts of ethanol production.
Grain ethanol plants are becoming less economical to operate due to lack of demand for ethanol and to low profit margins when grain prices are high and petroleum prices are low. Ethanol derived from grain is also criticized for having poor compatibility with fuel distribution systems, reducing the food supply, contributing to soil erosion, and releasing net CO2 emissions that are only marginally better than gasoline. Reduced operating costs, increased process efficiency, better fuel compatibility, and higher product value and diversity could significantly improve the economics and environmental acceptability of this process.
In a conventional dry mill process, whole grain is hammer milled, then steam treated in a jet cooker as it is sent to the fermentation tank for liquefaction with a thermostable alpha-amylase. Following cooling and saccharification, the mash is inoculated for fermentation. Variations on this basic process can involve separation of corn hulls (fiber), starch and germ gluten (protein) prior to saccharification, use of less steam for cooking, use of raw starch, recovery of edible corn oil from the germ and other changes. Following fermentation, ethanol is recovered by distillation, and the bulk of the fiber and protein, along with yeast cells and corn oil, are separated from the dissolved organics by centrifugation. This yields wet cake or distiller's wet grain (DWG) solids and thin stillage (TS) solubles. In a conventional process, the distiller's wet grain is dried to make distiller's dried grain (DDG) and the thin stillage containing the solubles (S) is evaporated to make a syrup, which is sprayed back onto the distiller's dried grain to make DDGS. Evaporation of the thin stillage separates a fraction of the inedible corn oil, which can be recovered for biodiesel production (FIG. 1).
Stillages (vinasse) following distillation of ethanol from industrial ethanol fermentations of grain include corn gluten and yeast protein, residual corn fiber, yeast cells, corn oil, and dissolved organics. Thin stillage contains significant quantities of glycerol (14 to 20 g/l), glucose disaccharides (e.g., cellobiose, trehalose, etc.) (6 to 10 g/l), xylose, lactic acid, corn oil and various oligosaccharides derived from residual undigested starch, dextrins, cellulose and hemicellulose. The total dissolved and suspended organic content of thin stillage is about 10% w/v. Table 1 presents a published summary of stillage components (Kim et al. 2008).
TABLE 1Exemplary Stillage CompositionStillage Componentg/lGlucose0.9Glucan (oligosaccharide)12.4Xylose0.7Xylan (oligosaccharide)3.7Arabinose0.4Arabinan (oligosaccharide)0.5Lactic acid16.8Glycerol14.4Acetic acid0.3Butanediol1.9Ethanol0.6
The glycerol and oligosaccharide contents of thin stillage retain water during evaporation and prevent drying. This makes thin stillage evaporation energy-inefficient. Removing glycerol and oligosaccharides prior to evaporation is therefore desirable. Stillages from other ethanol distillation processes also present disposal problems. Stillage is difficult and non-economical to treat in a waste water system because of its high biological oxygen demand (BOD), its high organic content and low pH. Stillage can also have relatively high nitrogen and phosphorous contents, about 2 g/l and 130 mg/l respectively (Yen et al. 2012). The massive volumes of thin stillage resulting from fuel ethanol production are particularly difficult to handle. A significant fraction of the thin stillage is therefore recycled or “backset” into the liquefaction stage, which increases the level of dissolved organics in the fermentation and whole stillage.
Methods and tools for converting on-site low value soluble organic stillage byproducts from ethanol production into biodiesel are needed to increase fuel production without harvesting more grain. Such methods and tools would reduce the organic load in the backset while creating higher value products such as yeast oil, enzymes, and animal feed from underutilized organic byproducts.